The invention relates to a method and apparatus for regulating gas pressures and more particularly to a method and apparatus for regulating the carrier gas pressure in a valveless column arrangement.
Valveless separation column arrangements have two successive separations columns. In order to backwash the system, the direction of flow of the gas in the first separation column can be reversed by a pressure gradient that is produced by switching the carrier gas feed. Such a procedure is disclosed in FIG. 1 of "Instrument News", Vol. 19, 1969, No. 4, page 16. The carrier gas is supplied to the inlet of the first separation column via a sample injector and then to the junction between the two separation columns (i.e., to the outlet of the first column). The pressures in the two carrier gas feeds are adjustable by pressure regulators. When used in laboratory gas chromatography the instruments are operated at relatively constant ambient temperatures, while in process gas chromatography the instruments may be exposed to temperature fluctuations. Commercial, temperature-compensated gas pressure regulators have significant temperature fluctuations. For example, at an exit pressure of 2 bars and at room temperature (20.degree. C.), there is an error of about 0.3% per 10.degree. C. Furthermore, this error disproportionately increases at higher pressures. For example, the error at 3 bars is about 0.6% per 10.degree. C.
The differential pressure decreasing in the valveless separation column arrangement over the first separation column determines the retention times of the column. Therefore, the differential pressure should be maintained as constant as possible.
Due to the differing fractional temperature errors occurring at different pressures, changes in the ambient temperature cause a variation in the differential pressure over the first column. Consequently, these differential pressure variations lead to retention time shifts, which because of the narrow peaks have an adverse effect on the desired result, particularly when high-performance capillary separation columns are used.
Accordingly, the problem in the prior art is the differential pressure acting in the first separation column is dependent of temperature and pressure-related deviations so that the retention times, which depend on the flow through the column, remain substantially constant.